The invention relates to an electronic device for regulating the voltage across a high-side load, especially for regulating a fan in a motor vehicle.
Such control devices are generally known through their use in motor vehicles.
There is known through JP 01302409 AA or DE 2708021 C3 an electronic control device in which a control voltage referenced to the positive high-side supply voltage is used as the command variable for the regulation.
An important fundamental function of the control device is to regulate the motor voltage in dependence on a control signal. That control signal may be an analogue control voltage, an analogue control current or a digital signal. Control devices for fans in accordance with the teaching of the prior art convert control currents and digital control signals internally into a control voltage, so that in principle there is always a control circuit that regulates the motor voltage as a function of a control voltage.
Known linear control devices for fans use in general the circuit topology illustrated in FIG. 2. A vehicle battery V2 provides the power supply for the entire arrangement. V1 provides the control voltage. Umot is the motor voltage. An operational amplifier U1A sets its output voltage and hence the gate-source voltage of the MOS transistor M1 in such a way that U+ is approximately equal to U−. The arrangement may be described by suitable equations. If R1/R2=R3/R4 is selected, the following relationship is obtained for control voltage V1 and motor voltage Umot:
      U    mot    =            V      1        ·                  R        2                    R        1            
An important requirement to be met by a control device for a fan is the correction of on-board voltage fluctuations. Umot should be independent of V2. That applies only when R1/R2=R3/R4. The behaviour of the control device shown in FIG. 2 is therefore dependent on the matching tolerance of the voltage dividers R1/R2 and R3/R4 in the case of on-board voltage fluctuations dV2.
Assuming ideal components and ideal matching tolerance R1/R2=R3/R4, the relationship Umot=f(V1) is determined only by the resistance ratio R1/R2. Umot is largely independent of V2. The operational amplifier corrects on-board voltage fluctuations. In standby operation, V1=0. The current consumption Ib of the arrangement is in this case described as closed-circuit current and should be as low as possible so as not to discharge the battery V2.
In the case where V1=0, Umot=0 and hence also Id=0 (modern mosfets have very small is cut-off currents). Accordingly, Ib=I1+I2+I3. I3 can be kept at a very low level by the use of an ultra-low-power opamp.
If it is desired for reasons of costs to dispense with encapsulation of the controller electronics, then the use of high-resistance resistors is problematic. Condensation and the associated contamination on the printed circuit board surface, which occur in the vehicle, lead to tracking currents which affect the functioning of circuits dimensioned to be of high-impedance. R1 to R4 cannot, therefore, be made to be high-resistance to an arbitrarily high degree. Thus, I1 and I2 load the battery in standby operation.
The problem underlying the invention is to develop a control device that permits relatively low-resistance resistors to be used even in the case of low closed-circuit current consumption, while compensating for thermal effects on the command variable.